The present invention relates to a cardiac biopotential analysis system and more particularly to a cardiac biopotential analysis system utilizing bispectral analysis to determine in a noninvasive manner, important myocardial physiologic properties.
Over the years many approaches have been utilized to extract information from the electrocardiogram regarding ischemia, propensity to ventricular tachycardia and other disorders in the heart which affect cardiac electrical activity. Most of these techniques have been restricted to analysis in the time domain (e.g., Cardiointegram.RTM., ST segment analysis, signal averaged late potentials). More recently, analysis involving quantification of the frequency content of portions or of all of the QRST complex as determined by the Fourier transform of a second order correlation function (better known as the power spectrum) has been utilized. These techniques have generally been found to work for certain purposes but not for others or have been found to generate figures of merit with poor positive predictive capability for the disorder evaluated.
Since cardiac signals arise from the discharge of hundreds of thousands of electrically active cells, these potentials produce a complicated resultant electrical signal. Imbedded in that signal is information regarding frequency content, non-linearities and phase relationships, all arising from the complex conduction dynamics that take place between the various regions of the cardiac tissue. When transmitted to the surface, where the cardiac signal is picked up by the electrodes, the cardiac signal undergoes alteration in morphology and frequency content as a result of factors including body fat content, rib cage size, and position of the heart relative to the lungs. All these variables lead to challenging signal processing problems that conventional time and frequency domain analyses fail to address, since information regarding non-linearities and phase relationships is suppressed.
The electrical signal of the normal heart is a composite of the multitude of individual signals which are repetitively active in a relatively synchronized, organized pattern. Coronary artery disease may lead to episodes of ischemia, which alter the normal pattern of electrical activity of the heart. The conventional scalar ECG displays ischemia as a shift in the ST segment, but this technique is not sensitive or specific enough to allow a reliable noninvasive diagnosis of coronary disease.
In those patients with coronary atherosclerosis, especially in conjunction with heart damage and scarring, there is an enhanced risk of sudden cardiac death from ventricular arrhythmias. Although ventricular tachycardia (VT) can often be the initiating arrhythmia, the terminal arrhythmia is ventricular fibrillation (VF). The propensity for VF is determined by the degree of heterogeneity or disorganization of cardiac electrical conduction and repolarization. Such heterogeneity, however, cannot be discerned using conventional scalar electrocardiography. Therefore, the ability to predict noninvasively, reliably and quantitatively this propensity for VF would be highly desirable, and the further ability to predict VT, an initiator of VF, would enhance overall predictive power.
Reentrant circuits capable of sustaining ventricular tachyarrhythmias require heterogeneous conduction pathways, a substrate commonly occurring in individuals with myocardial infarction. Delayed ventricular activation is often seen as an electrophysiological consequence of myocardial infarction. The detection of delayed ventricular potentials identifies a subset of patients at higher risk for ventricular tachyarrhythmias, as compared to those patients not having late potentials. Although the mere presence of late potentials is highly sensitive for ventricular tachyarrhythmias (i.e., it correctly detects many of those who will develop ventricular tachyarrhythmias), it is not highly specific (i.e., it falsely detects many of those who will not develop ventricular tachyarrhythmias).
It is therefore a principal object of the present invention to provide a noninvasive system and method for diagnosing coronary disease.
A further object of the present invention is to provide a noninvasive system and method for the detection and quantification of myocardial ischemia.
Another object of the present invention is to provide a noninvasive system and method for the detection and quantification of cardiac electrical instability.
It is still another object of the present invention to provide a noninvasive system and method for quantifying heterogeneity in conduction and repolarization.
A still further object of the present invention is to provide a cardiac biopotential analysis system and method which obtains information in a noninvasive fashion that is comparable to information obtained through invasive electrophysiologic testing and coronary angiography.